Distributed amplification is the amplification of portions of a signal along the length of a first (input) transmission line by discrete active elements (amplifiers), the outputs of which are combined along a second (output) transmission line to produce the resultant amplified signal.
Each active element in a distributed amplifier contains spurious reactances, mostly capacitive, which impact upon the input and output impedances of those devices. A properly designed circuit containing these active elements compensates for these spurious reactances so that they will have minimum effect upon the transfer of power at the desired operating frequencies. Traditionally, this is accomplished by coupling both the inputs and the outputs of the active elements to compensatory networks. Such networks contain both inductances and capacitances along their lengths so as to appear as short lengths of transmission lines having specific characteristic impedances and operating over wide frequency bands. Each of the input and output networks may therefore be considered a virtual transmission line. In this approach, the input and output terminals of the active elements are connected at regular intervals along the input and output virtual transmission lines, respectively.
In the above and related implementations, the input and output virtual transmission lines are designed to have constant characteristic impedances over their lengths. This allows the theoretical gain of the distributed amplifier to approach the sum of the gains of the individual active elements. Simultaneously, the upper limits of the amplifier bandwidth are extended to the point where the spurious capacitance compensation no longer holds true. This results in a distributed amplifier that retains the wideband performance of the virtual transmission lines.
Although distributed amplifiers are highly attractive for those applications requiring a high gain-bandwidth product, they are not so well suited for applications requiring high output power. This is because the output power of a uniformly distributed amplifier is limited to the power-handling capability of the final active element in the amplifier. Each active device in a distributed amplifier contributes an equal output current to the output virtual transmission line. These currents progressively accumulate in the direction of the load. Since the characteristic impedance of the output virtual transmission line is constant throughout its length, the voltage across each successive active element must be greater so that the voltage-to-current ratio remains constant. Therefore, a conventional uniformly distributed amplifier is designed so that the final active element operates optimally within its rated voltage and current specifications. All preceding active elements accordingly operate suboptimally.
Additionally, conventional distributed amplifiers operate with matched-load power transfers. Since the virtual transmission lines appear as true transmission lines to the circuit, they have a characteristic impedance that needs be terminated to prevent interference from signal reflection. Since one of the purposes of the virtual transmission lines is to compensate for the spurious capacitive reactance of the active elements, they are themselves predominantly reactive. Termination becomes increasingly important with reactive characteristic impedances. In a terminated transmission line, approximately one-half the power is dissipated in the terminating impedance. Since there are typically both an input and an output virtual transmission line, only one-quarter of the output power is available for a given input signal.
Also of significance in traditional distributed amplifiers is odd-harmonic generation. Since each active element amplifies a portion of the input signal at a different phase angle, the output signal is comprised of the sum of partial-amplification products. Theoretically, the output virtual transmission line delays each partial product by just the proper amount so that all partial products are in phase when summed. In practice, this leads to a distortion of the output signal, hence an in-phase summing of odd harmonics. This produces significant third-harmonic distortion. Various techniques, some quite complex, are used to curtail this third-harmonic distortion.
The current state of the art in the manufacture of monolithic power field-effect transistors or other "active elements" lends itself to pairs of such transistors per substrate. As these transistor pairs are ideally suited for power output applications, the implementation of two-stage distributed amplifiers rises correspondingly in importance. Traditional distributed amplifier implementations gain efficiency and performance only with more than two stages. With more than two stages, more than one pair of monolithic transistors is required. Since the resultant active elements do not all share the same substrate, they thus do not share the same spurious capacitance values, gain, etc. These factors may cause a decrease in performance and/or an increase in design complexity.